1. Field of the Invention
The present invention relates to an EL panel in which an EL element formed on a substrate is sealed between the substrate and a cover member. The invention also relates to an EL module with IC mounted to the EL panel. In this specification, the EL panel and the EL module are generically called light emitting devices. The present invention further relates to an electronic device employing the light emitting devices.
2. Description of the Related Art
Self-light emitting EL elements eliminate the need for a backlight that is necessary in liquid crystal displays (LCDs) and thus make it easy to manufacture thinner displays. Also, the EL elements are high in visibility and have no limit in terms of viewing angle. These are the reasons for attention that light emitting devices using the EL elements have been receiving recently as display devices to replace CRTs and LCDs.
An EL element has a layer containing an organic compound that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as EL layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds includes light emission in returning to a base state from singlet excitation (fluorescence) and light emission in returning to a base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use both types of light emission.
All layers that are provided between an anode and a cathode are an EL layer in this specification. Specifically, the EL layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, an electron transport layer, etc. A basic structure of an EL element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transport layer, and a cathode layered in this order.
The EL element defined herein is a light emitting element that is composed of an anode, an EL layer, and a cathode. In this specification, an EL element emitting light is expressed as an EL element being driven.
Methods of driving a light emitting device comprising an EL element are roughly divided into analog driving methods and digital driving methods. Digital driving is deemed to be more promising in view of transition from analog broadcasting to digital broadcasting since it enables the light emitting device to display an image using a digital video signal that has image information as it is without converting the signal into an analog signal.
Described below is the structure of a pixel portion in a common light emitting device driven by a time division driving method. The description is given with reference to FIG. 17.
FIG. 17 is a circuit diagram of a pixel portion in a common light emitting device. A pixel portion 9001 has source signal lines S1 to Sx, power supply lines V1 to Vx, and gate signal lines G1 to Gy. The pixel portion 9001 includes a plurality of pixels 9002 that form a matrix.
Each of the pixels 9002 has one of the source signal lines S1 to Sx, one of the power supply lines V1 to Vx, and one of the gate signal lines G1 to Gy. Each of the pixels 9002 also has a switching TFT 9003, an EL driving TFT 9004, and an EL element 9006.
The switching TFT 9003 has a gate electrode connected to one of the gate signal lines G1 to Gy. The switching TFT 9003 has a source region and a drain region, one of which is connected to one of the source signal lines S1 to Sx and the other of which is connected to a gate electrode of the EL driving TFT 9004 and to a capacitor 9005 that is provided in each of the pixels 9002.
The capacitor 9005 is provided to hold the gate voltage (the difference in electric potential between the gate electrode and a source region) of the EL driving TFT 9004 when the switching TFT 9003 is riot selected (when the TFT 9003 is in an OFF state).
The source region of the EL driving TFT 9004 is connected to one of the power supply lines Vi to Vx whereas a drain region thereof is connected to the EL element 9006. The power supply lines V1 to Vx are respectively connected to the capacitors 9005 in the pixels.
The EL element 9006 is composed of an anode, a cathode, and an EL layer placed between the anode and the cathode. If the anode is connected with the drain region of the EL driving TFT 9004, the anode serves as a pixel electrode whereas the cathode serves as an opposite electrode. On the other hand, the cathode serves as the pixel electrode whereas the anode serves as the opposite electrode if the cathode is connected with the drain region of the EL driving TFT 9004.
The opposite electrode of the EL element 9006 is given an electric potential (opposite electric potential) from a power supply external to the EL panel. The power supply lines V1 to Vx are also given an electric potential (power supply electric potential Vp) from a power supply external to the EL panel.
The operation of the pixel portion 9001 shown in FIG. 17 is described next.
A selection signal is inputted to the gate signal line G1 to select the gate signal line G1 and turn every switching TFT 9003 whose gate electrode is connected to the gate signal line G1 ON. In this specification, a signal line being selected means that every TFT whose gate electrode is connected to the signal line is turned ON.
Through the switching TFT 9003 that is turned ON, a digital signal which carries image information (hereinafter the signal is referred to as digital video signal) and which is inputted to the source signal lines S1 to Sx is inputted to the gate electrode of the EL driving TFT 9004.
The digital video signal inputted to the gate electrode of the EL driving TFT 9004 contains information, which is ‘1’ or ‘0’ and used to control switching of the EL driving TFT 9004.
When the EL driving TFT 9004 is turned OFF, the electric potential of the power supply lines V1 to Vx is not given to the pixel electrode of the EL element 9006 and therefore the EL element 9006 does not emit light. On the other hand, when the EL driving TFT 9004 is turned ON, the electric potential of the power supply lines V1 to Vx is given to the pixel electrode of the EL element 9006 to cause the EL element 9006 to emit light.
When the gate signal line G1 is no longer selected, the gate signal line G2 is selected to repeat the operation described above. An image is displayed when the gate signal lines G1 to Gy are sequentially selected until all of them are selected once and the above operation is conducted in every pixel.
In the driving method described above, the power supply electric potential Vp given to each power supply line by the power supply external to the EL panel is given to the source region of the EL driving TFT 9004 of each pixel. Ideally, the same level of electric potential Vp is given to the source region of every EL driving TFT 9004 that is connected to the same power supply line.
In fact, however, a power supply line has its own resistance (wiring line resistance) to make the electric potential vary over the length of the power supply line. Due to the wiring line resistance, the electric potential of a power supply line becomes closer to the electric potential of a ground and the difference from the power supply electrics potential Vp is increased as the distance from the power supply is increased. Accordingly, the electric potential given to the source region of one EL driving TFT 9004 is different from the electric potential given to the source region of another EL driving TFT 9004 depending on the site at which the TFT is connected to the power supply line even though the TFTs are connected to the same power supply line.
The difference in electric potential between different sites of one power supply line is greater when the amount of current flowing into the power supply line is larger. In other words, even though the distance from the power supply is the same, the electric potential difference due to wiring line resistance becomes greater and the electric potential at the site becomes much closer to the electric potential of a ground than the power supply electric potential Vp as the amount of current flowing into the power supply line is increased.
The amount of current flowing into a power supply line is varied depending on an image to be displayed. This is because the ratio of pixels that emit light and the ratio of pixels that do not emit light to all the pixels, that share the same power supply line, vary between images. When an image to be displayed requires more pixels that emit light than pixels that do not emit light, the amount of current flowing into the power supply line is larger and the difference in electric potential among different sites of the power supply line is greater. On the other hand, when an image to be displayed requires more pixels that do not emit light than pixels that emit light, the amount of current flowing into the power supply line is smaller as well as the difference in electric potential among different sites of the power supply line.
The difference in electric potential given to source regions makes an electric potential given through one EL driving TFT 9004 to the pixel electrode of one EL element 9006 different from an electric potential given through another EL driving TFT 9004 to the pixel electrode of another EL element 9006. The amount of current flowing into an EL element is different from the amount of current flowing into another EL element whose pixel electrode is connected through the EL driving TFT 9004 to the power supply line to which the former EL element is connected. Therefore EL elements connected to the same power supply line emit light with different luminance in accordance with positions at which the EL elements are connected to the power supply line. The term luminance herein means brightness of an EL element per unit area at the instant the EL element emits light.
The difference in luminance among pixels is greater when the difference in electrics potential over the length of a power supply line is greater.
FIGS. 18A and 18B are schematic diagrams of gray scale of pixels in a pixel portion. In FIGS. 18A and 18B, the pixel portion has nine pixels for the sake of simple explanation.
A pixel (1, 1), a pixel (1, 2), and a pixel (1, 3) have the same power supply line V1. In other words, pixel electrodes of EL elements of the pixel (1, 1), the pixel (1, 2), and the pixel (1, 3) are connected to the same power supply line V1 through EL driving TFTs. A pixel (2, 1), a pixel (2, 2), and a pixel (2, 3) have the same power supply line V2. A pixel (3, 1), a pixel (3, 2), and a pixel (3, 3) have the same power supply line V3.
Source regions of EL driving TFTs of the pixel (1, 1), the pixel (2, 1), and the pixel (3, 1) are respectively connected to the power supply lines V1, V2, and V3 on the closest side to the power supply.
Consider a case where all the pixels are to emit light with the same intermediate gray scale. The same amount of current flows into the power supply lines V1, V2, and V3. Due to the wiring line resistance, the electric potential of a power supply lines becomes closer to the electric potential of a ground as the distance from the power supply is increased. Accordingly, the pixel (1, 1), the pixel (2, 1), and the pixel (3, 1) are the brightest whereas the pixel (1, 3), the pixel (2, 3), and the pixel (3, 3) are the darkest.
In this case, however, the difference in luminance between adjacent pixels is not great enough to be recognizable by the human eye. Also, although the difference in luminance is the greatest between the nearest pixel to the power supply of a power supply line and the farthest pixel from the power supply, the human eye hardly detects the difference in luminance between pixels apart from each other.
Next, consider a case where all pixels except the pixel (2, 2) are to emit light with the same intermediate gray scale. The current flowing into the power supply line V2 is smaller than the current respectively flowing into the power supply lines V1 and V3. Therefore, the difference in electric potential over the length of the power supply line V2 is smaller than those of the power supply lines V1 and V3.
As the difference in electric potential over the length of a power supply line becomes smaller, the electric potential of the power supply line becomes closer to the power supply electric potential Vp than the electric potential of a ground. Then, the difference in electric potential between a pixel electrode of an EL element and an opposite electrode of the EL element is increased to increase the amount of current flowing into the EL element and raise the luminance of pixels that have this power supply line.
Accordingly, the luminance of the pixel (2, 1) is higher than the luminance of the pixel (1, 1) and the pixel (3, 1) as shown in FIG. 18A. The luminance of the pixel (2, 3) is higher than the luminance of the pixel (1, 3) and the pixel (3, 3).
The human eye has difficulty in detecting the difference in luminance between pixels apart from each other. Therefore, the difference in luminance between the pixel (1, 1) or the pixel (3, 1) and the pixel (1, 3) or the pixel (3, 3) is not so obvious to the human eye. However, a large difference in luminance between adjacent pixels is noticeable and easily recognized by the human eye. The difference in luminance between the pixel (2, 1) and the pixel (1, 1), or the pixel (2, 1) and the pixel (3, 1), is obvious to the human eye, as well as the difference in luminance between the pixel (2, 3) and the pixel (1, 3), or the pixel (2, 3) and the pixel (3, 3).
Another case is considered in which the pixel (2, 2) emits light with the highest luminance while the rest of the pixels all emit light with intermediate gray scale. In this case, the amount of current flowing into the power supply line V2 is larger than the amount of current respectively flowing into the power supply lines V1 and V3. The difference in electric potential over the length of the power supply line V2 is accordingly greater than those of the power supply lines V1 and V3.
As the difference in electric potential over the length of a power supply line becomes greater, the electric potential of the power supply line becomes closer to the electric potential of a ground than the power supply electric potential Vp. Then, the difference in electric potential between a pixel electrode of an EL element and an opposite electrode of the EL element is decreased to reduce the amount of current flowing into the EL element and lower the luminance of pixels that have this power supply line.
Accordingly, the luminance of the pixel (2, 1) is lower than the luminance of the pixel (1, 1) and the pixel (3, 1) as shown in FIG. 18B. The luminance of the pixel (2, 3) is lower than the luminance of the pixel (1, 3) and the pixel (3, 3).
Similar to the case illustrated in FIG. 18A, the human eye has difficulty in detecting the difference in luminance between pixels apart from each other. Therefore, the difference in luminance between the pixel (1, 1) or the pixel (3, 1) and the pixel (1, 3) or the pixel (3, 3) is not so obvious to the human eye. However, a large difference in luminance between adjacent pixels is noticeable and easily recognized by the human eye. The difference in luminance between the pixel (2, 1) and the pixel (1, 1) or the pixel (2, 1) and the pixel (3, 1) is obvious to the human eye, as well as the difference in luminance between the pixel (2, 3) and the pixel (1, 3) or the pixel (2, 3) and the pixel (3, 3).
The phenomena shown in FIGS. 18A and 18B is called crosstalk. Crosstalk takes place more often as the area of the pixel portion is increased and the wiring line resistance of the power supply lines is raised.